Field of the Invention
The present invention relates to clutch protection control of an automatic transmission.
Description of the Related Art
Japanese Patent Laid-Open No. 2011-27210 discloses clutch protection control that controls the engagement of a clutch to a permission state, a partial permission state, or an inhibition state based on the temperature, cooling rate, and heat generation amount of the clutch at the time of gear change of an automatic transmission. In a conventional automatic transmission in which input and output shafts (a main shaft and a counter shaft) are arranged in parallel, the clutch lubrication amount and the main shaft rotation speed hold an almost proportional relationship, as shown in FIG. 8A, and the clutch lubrication amount and the clutch cooling rate hold an almost proportional relationship, as shown in FIG. 8B. That is, as shown in FIGS. 8C and 8D, the main shaft rotation speed can generally be regarded as an alternative parameter of the clutch cooling rate. It is therefore possible to calculate an allowable clutch temperature from the main shaft rotation speed and execute clutch protection control based on the relationship between the clutch temperature and the allowable clutch temperature.
However, since an automatic transmission that establishes a gear range by a plurality of planetary gear mechanisms has a structure in which a plurality of constituent components (sun gears, planetary gears, and ring gears) rotate relative to main shaft rotation, the clutch lubrication amount does not have a simple proportional relationship to the main shaft rotation speed, as shown in FIG. 9A. In addition, even if the input shaft rotation speed remains unchanged, the clutch lubrication amount changes when the gear range changes. Furthermore, as for the cooling rate with respect to the clutch lubrication amount as well, the cooling rate changes in accordance with the operation state (release state/engaging state/guide rotation stop state) of the clutch, as shown in FIG. 9B. For this reason, it is impossible to obtain an allowable clutch temperature suitable for an actual situation from the main shaft rotation speed as in the prior art.